The expansion of stem cells is an essential, highly regulated process during development and adult tissue homeostasis. Deregulation of this process results in numerous diseases, including developmental syndromes, diabetes, osteoporosis, neurodegeneration, premature aging, and cancer. While great strides have been made in defining the signaling pathways that mediate activation of normally quiescent adult stem cells, highlighting WNT/b-catenin signaling as being key in this process, there is a significant gap in knowledge concerning the factors within stem cells that respond to these extrinsic niche factors. Bridging this knowledge gap is essential to advance the use of adult stem cells in regenerative medicine, and to identify new druggable targets for diseases marked by deregulated adult stem cell activation. We are using the mammary gland as a highly tractable model system to elucidate the transcriptional mechanism mediating adult stem cell activation. The goal of this study is to identify the in vivo mechanistic action of LBH, a novel and highly unusual WNT transcriptional regulator that we have shown is expressed in multiple WNT-dependent adult epithelial stem cell niches, and is aberrantly overexpressed in WNT-related cancers. Our hypothesis, based on preliminary evidence, is that LBH is a critical cell-intrinsic WNT effector of adult epithelial stem cell activation by inducing expression of the key epithelial stem cell transcription factor, deltaNp63, which results in a self-renewing multipotent stem cell state and represses cell differentiation. First, the stem cell populations and adult stages that require LBH for function will be determined through analysis of a unique conditional LBH knockout mouse model, we have generated. Both in vitro and in vivo stem cell assays based on transplantation of purified mammary stem cells into the mammary fat pads of murine hosts will be performed. Furthermore, we will characterize the stem cell-promoting effects of LBH through inducible overexpression of LBH in primary non- stem mammary epithelial cell populations. Second, we will determine the role of LBH in WNT-controlled mammary stem cell activation using genetic mouse models as well as clonogenic assays. Third, we will test, if deltaNp63 is directly regulated by LBH and if WNT signaling is required in this context. The functional significance of LBH activation of deltaNp63 in mammary stem cell control will be elucidated by introducing deltaNp63 into Lbh- deficient basal cell populations and assessing its ability to restore multipotent stem cell traits. The proposed work will yield critical new insights into the cell-intrinsic factors promoting the mobilization of adult stem cells. This knowledge will be essential to advance regenerative medicine, as well as to uncover new targets for the intervention of diseases caused by aberrant stem cell activity, notably cancer.